In a case in which a pneumatic actuator, such as a pneumatic cylinder or a pneumatic motor, is actuated by compressed air, using clean compressed air in which dust and liquid, such as oil contents and water contents, are not mixed is required. Accordingly, a pneumatic filter for removing foreign matters, such as dust and liquid, from the compressed air is typically used in a pneumatic circuit that supplies compressed air to the pneumatic actuator.
As disclosed in Patent Literature 1, for example, a pneumatic filter has a configuration in which a filter body for removing foreign matters is housed inside a filter case including an inlet port and an outlet port. The filter body includes a hollow filter member, and an upper cap and a lower cap attached to an upper end and a lower end of the filter member, the upper cap includes a communication hole for communicating the inlet port and the hollow portion of the filter member to each other, and the lower cap functions to close a lower end portion of the hollow portion of the filter member.
Furthermore, when compressed air from the inlet port is supplied into the hollow portion of the filter member through the communication hole of the upper cap, the compressed air is filtered while passing through the inside to the outside of the filter member removing the foreign matters, and the purified compressed air flows out from the outlet port.
Meanwhile, liquid, such as oil contents and water contents, separated from the compressed air gradually develops into large liquid particles by repeated merging of small particles while flowing down along the filter member, flows down to the inside of the lower cap after being temporarily retained therein, sequentially drips into the filter case from the lower cap, and is discharged as drain.
However, in conventional pneumatic filters, in a case in which the flow velocity of the compressed air is high, in portions near the lower end portion of the filter member and near the upper surface of the lower cap, liquid contained in the filter member at high density and liquid accumulated inside the lower cap come in contact with the fast flow of the compressed air and are scattered causing a problem in that the compressed air that has been purified is easily mixed again. In particular, when the liquid inside the lower cap flows over the lower cap, the liquid on the edge of the lower cap is easily scattered by the flow of compressed air.
Incidentally, Patent Literature 2 discloses a filter element that can overcome such a problem. The filter element disposes, on the inside and the outside in a dual manner, a hollow filter layer with a small diameter that removes dust, and a hollow drain layer with a large diameter that collects liquid, an upper cap and a lower cap are attached to upper ends and lower ends of the filter layer and the drain layer, and a lower end portion of the drain layer is extended so as to cover a portion of the lower cap from a lateral side of the outer periphery to an underside.
In the filter element, when liquid is collected in the drain layer while the compressed air passes through the filter tank to the drain layer, since the liquid flows downwards along the drain layer and drips from a drain layer portion positioned on the underside of the lower cap, there is no contact with the flow of the compressed air and, accordingly, mixing into the compressed air once more is prevented.
However, in the filter element, since the lower end portion of the drain layer needs to be formed into a special shape covering a portion of the lower cap from the outer periphery of the lateral side to the underside, there is a drawback in that work and cost are required in manufacturing the filter element. When deforming a uniform cylindrical drain layer into the above shape, wrinkles easily occur such that the above work is very troublesome.